Blood-borne peroxidase leaks out of capillaries and is taken up by axon terminals innervating muscle beds and specific circumventricular organs (i.e. median eminence, neurohypophysis and area postrema). The peroxidase is transported intra-axonally from axon terminal to parent cell body by retrograde axoplasmic flow. The central nervous system neurons which are found to be peroxidase positive include all cranial and spinal cord motor nuclei, hypothalamic neurosecretory cells (i.e. paraventricular, supraoptic, arcuate, periventricular, catecholamine- containing A1 and A5 cell groups in the medulla and pons) and the nucleus of the tractus solitarius which is afferent to the area postrema.